1. Field of the Invention
The present invention relates to a multi-stage variable speed compressor provided with a plurality of compressors.
2. Description of the Related Art
Recently, a multi-stage variable speed compressor has been known, of which an object is to obtain a desired pressure by serially connecting a plurality of compressors to increase pressure step by step, when the desired pressure cannot be obtained using one compressor. A method for controlling such a multi-stage variable speed compressor has already known from Japanese Laid-open Patent Publication No. Hei 10-82391. This technique is to obtain said target discharge pressure by setting a suction flow rate and a target discharge pressure in a multi-stage variable speed compressor provided with a two-stage compressor.
The control of this multi-stage variable speed compressor is intended to obtain the desired discharge pressure, in such a manner that values to be taken for each part of the multi-stage variable speed compressor, which are corresponding to said setting (for example, a mid-pressure between the two compressor stages, a rotation ratio of the two compressors, the rotational speed and etc.), are previously stored in a control device as operation data, and the control device of the multi-stage variable speed compressor determines operation data corresponding to said setting and controls the multi-stage compressor, so that each part of the multi-stage variable speed compressor is laid to these operation data values.
However, because a multi-stage variable speed compressor is subject to a secular change (for example, loss of gas-tightness, abrasion of bearing parts or the like) while being used and thus its specifications are changed, it may be impossible to obtain a desired pressure even if each part of the multi-stage variable speed compressor is controlled using operation data previously stored as explained in the above.
In addition, it is frequent that a compressor (including a multi-stage) is usually intended to obtain a desired discharge pressure. Accordingly, if a flow rate of working fluid is changed during the operation, the rotational speed of the compressor or the like should be changed in response to the change of flow rate so as to obtain said desired discharge pressure.
For that reason, if the control of a multi-stage variable speed compressor is performed using operation data as in the prior art, it is necessary to prepare operation data corresponding to a flow rate-changing width of working fluid for each of discharge pressures that the multi-stage variable speed compressor can output.
In particular, if it is possible for the multi-stage variable speed compressor to be controlled up to 75% of a flow rate-changing width with a 1% pitch of changing width for one discharge pressure, it is necessary to prepare 75 kinds of operation data. In addition, if it is possible to operate said multi-stage variable speed compressor at a discharge pressure in the range of 2 to 10 kg/cm2 with a 0.1 kg/cm2 pitch of pressure, it is necessary to prepare 80 kinds of operation data. Then, it is required to prepare 6000 (75xc3x9780=6000) kinds of operation data for the multi-stage variable speed compressor and is necessary to prepare a large amount of operation data.
Furthermore, if the multi-stage variable speed compressor is subject to a secular change as explained in the above said large amount of operation data cannot be used. Therefore, it is difficult to say that the prior art can endure a practical use.
Therefore, the present invention is conceived considering the above situation. It is an object of the present invention to provide a multi-stage variable speed compressor, wherein the compressors from the first stage to the final stage are controlled to obtain the desired target pressure.
In order to achieve the above object, the present invention provides a multi-stage variable speed compressor comprising:
a plurality of compressors of which the rotational speeds are variable and which are serially connected each other so that working fluid is compressed step by step;
a pressure detector for detecting a discharge pressure of the compressor of the final stage among said plurality of compressors; and
a control device for controlling the rotational speeds of said plurality of compressors so that the discharge pressure of the compressor of said final stage among said plurality of compressors is laid to a desired target pressure,
wherein said control device inputs a pressure signal detected by said pressure detector, calculates a deviation between said target pressure and said pressure signal, and controls the rotational speeds of said plurality of compressors using said deviation.
As constructed in this manner, the discharge pressure is always controlled to be the desired target pressure. Accordingly, it becomes possible to operate the multi-stage variable speed compressor with a discharge pressure of a good precision. In addition, it becomes possible to perform the control without needing operation data as used in the prior art at all.
The multi-stage variable speed compressor according to the present invention, further comprises a pressure detector provided for each of compressors on and after the second stage among said plurality of compressors and detecting a discharge pressure of a compressor of a stage preceding the corresponding compressor or a suction pressure of the corresponding compressor, wherein said control device can be constructed to control the rotational speed of the compressor of the first stage among said plurality of compressors using said deviation, to input a pressure signal from said pressure detector provided for each of compressors on and after the second stage among said plurality of compressors and detecting a discharge pressure of a compressor of a stage preceding the corresponding compressor or a suction pressure of the corresponding compressor, and to control the rotational speed of each compressor on and after the second stage among said plurality of compressors based on the discharge pressure of the compressor of a stage preceding the corresponding compressor or the suction pressure of the corresponding compressor.
Herein, it is preferred that said control device calculates the rotational speed of the compressor of the first stage among said plurality of compressors by PID (Proportional Integral Derivative) operation rather than the deviation of the discharge pressure of the compressor of the final stage from the target pressure. It is also preferred that said control device calculates the rotational speed of each of compressors on and after the second stage from a discharge pressure of a compressor of a stage preceding the corresponding compressor or a suction pressure of the corresponding compressor by PID operation.
In addition, said discharge pressure of a compressor of a stage preceding the corresponding compressor and the suction pressure of the corresponding compressor are substantially equal if a structure such as a throttle valve or the like which causes a change of pressure is not interposed between the preceding compressor and the corresponding compressor.
According to said construction, the control for allowing the discharge pressure of the final stage to be laid to the desired target pressure is performed with a good precision.
Also, the multi-stage variable speed compressor according to the present invention further comprises a pressure detector for detecting a discharge pressure of the corresponding compressor with regard to compressors except the final stage compressor among said plurality of compressors, wherein said control device can be constructed in such a manner as to feed-back control the rotational speed of a compressor of each stage based on a pressure deviation between a target discharge pressure given to a compressor of each stage and a pressure from the pressure detector for detecting said discharge pressure of each stage, for all of said plurality of compressors. Herein, it is preferred that said target pressure of the discharge pressure given to each stage is determined in connection to said target pressure of the discharge pressure of the final stage compressor.
Herein, it is preferred to construct in such a manner that said target pressure of the discharge pressure given to each stage is determined using said target pressure of discharge pressure of the final stage and a compression ratio of a compressor of each stage.
Further, in the multi-stage variable speed compressor according to the present invention, it is possible to construct said control device in such a manner as to determine the rotational speed of each compressor based on said deviation, for all of said plurality number of compressors, for all of said plurality of compressors, wherein the relationship between the rotational speeds of respective stages is previously set.
For example, it is possible to construct said control device in such a manner as to determine the rotational speed of the compressor of the first stage among said plurality of compressors based on said deviation, and to determine the rotational speed of each of the compressors on and after the second stage among said plurality of compressors based on data that previously set the relationship between the rotational speed of the compressor of the first stage and the rotational speed of each of the compressors on and after the second stage. Herein, it is preferred to construct in such a manner that the rotational speed of the compressor of the first stage is determined by performing PID operation of said deviation.
According to this construction, the rotational speeds of compressors of respective stages can be determined instantaneously at the same time, whereby the control has an immediate response characteristic.
Further, in the multi-stage variable speed compressor according to the present invention, it is possible to construct said control device in such a manner as to calculate a change rate of the rotational speed of compressor in the first stage among said plurality of compressors, and to be switched to determine the rotational speed of each compressor based on said deviation if said change rate is larger than a predetermined threshold value, for all of said plurality of compressors, wherein the relationship between the rotational speeds of respective stages are previously set.
According to this construction, the multi-stage variable speed compressor is controlled with a good precision by sequentially controlling the rotational speed of each compressor based on a discharge pressure of a compressor in a stage preceding the corresponding compressor or the suction pressure of the corresponding compressor if said change rate is small, while the control immediately responsive to the change is performed by determining the rotational speed of each compressor instantaneously at the same time based on the relationship between the rotational speed of said each stage if said change rate is large. In this manner, the control methods are precisely classified and used, as occasion requires, whereby the control capability of the multi-stage variable speed compressor is enhanced.
It is possible to construct the multi-stage variable speed compressor according to the present invention in such a manner that all or some of suction ports of said plurality of compressors are connected each other and each of interconnected conduit lines is provided with a control valve.
According to this construction, it is possible that the discharge pressure of the multi-stage variable speed compressor is laid to a target pressure using some of the compressors when it is not required to operate all of the compressors to obtain the desired target pressure. Due to this, it is economical because electric power to be consumed is suppressed as compared to the case that all of the compressors are operated.
In the multi-stage variable speed compressor according to present invention, it is possible to construct said control device in such a manner as to automatically stop said multi-stage variable speed compressor, when said multi-stage variable speed compressor is operated below the rotational speed at which it can normally operate.
Generally, in a compressor, if the rotational speed of a compressor is low, the working fluid may flow backward or stay within the compressor once compressed. In this case, there is problem in that if the working fluid is compressed again, the working fluid becomes a high temperature and aversely affects machines. However, the construction as explained in the above can solve said problem.